The present invention generally relates to mechanical fastening and, more particularly, to measurement of the pin protrusion from the bearing surface of the nut to the end of the bolt for a fastening using a nut and bolt.
In the assembly line manufacture of commercial aircraft, nut and bolt fasteners that fasten various components of the airframe and aircraft structure may require the nuts to be fastened in relatively inaccessible locations or confined spaces, such as the aft side of the aft pressure bulkhead in the tail cone area of the aircraft fuselage. FIG. 1 shows a typical fastener 100 including a bolt 102 or pin 102, nut 104, and washer 106 combination. Bolts 102 may be provided with various lengths—called the grip length of the bolt (not shown)—depending on the thickness of structure which is to be held together by the nut-bolt combination and through which the bolt may pass. Different bolts may have a fixed thread length for various grip lengths. In other words, a long bolt, such as pin 102 shown in FIG. 1, may have threads cut for a certain length, the thread length 108, along the bolt from the end of the bolt, and the thread length 108 may be the same for a longer bolt and for a shorter bolt. Because the fasteners may have a fixed thread length 108 for various grip lengths, fasteners are required to conform to specifications regarding the height that the bolt is allowed to protrude, referred to as “pin protrusion”.
More specifically, a maximum pin protrusion 110 may be the specified maximum height that pin 102 may protrude above the bearing surface 112 of nut 104. Maximum pin protrusion 110 may also be measured from the bearing surface 114 of washer 106, which is in contact with bearing surface 112 of nut 104. If the maximum pin protrusion 110 is exceeded, nut 104 could engage the last thread of the pin 102 before the required compression of the joint being fastened occurs. Nut 104 engaging the last thread of thread length 108 on the bolt 102 may also be described as the nut 104 “bottoming out” on the threads of bolt 102. A nut 104 that bottoms out may result in a false torque reading when tightening nut 104, and inadequate fastening of fastener 100. Conforming to the maximum pin protrusion 110 specification may ensure, for example, that all of the threads of nut 104 that are engaged with threads of pin 102 are engaged so as to be load bearing, and may ensure proper tightening of nut 104 and pin 102 of fastener 100.
A minimum pin protrusion 116 may be the specified minimum height that pin 102 may protrude above the surface 118 of aircraft structural component 120. Minimum pin protrusion 116 may also be referred to in the art as “minimum pin protrusion to avoid threads in bearing.” A minimum thread protrusion 122 may be the specified minimum height that pin 102 may protrude above nut 104. Conforming to the minimum thread protrusion 122 specification may ensure, for example, that an adequate number of threads of pin 102 and nut 104 are engaged in order for fastener 100 to bear the required load. Conversely, a minimum thread protrusion 122 or minimum pin protrusion 116 that is less than the specification may cause fastener 100 to fail.
Pin protrusions, particularly maximum pin protrusion 110, are commonly measured in the prior art using a hand-held gage which may be formed from a rectangular card with a notch cut out so that the gage resembles an L-shaped card. One leg of the L may be placed over the end 124 of pin 102 and the end of the other leg of the L may be aligned with bearing surface 114 of washer 106 in order to achieve a “fit”. The alignment of the gage with bearing surface 114 may be performed via mechanical contact of the gage with the bearing surface 114, if washer 106 projects sufficiently far from underneath nut 104 or, if not, the alignment may be performed visually by the user of the gage. Such a prior art hand-held gage is typically one of a set of gages of graduated sizes. If a gage that is larger than the maximum pin protrusion 110 specified, or one that is smaller than, for example, the minimum pin protrusion 116 or minimum thread protrusion 122 specified, can be made to “fit” as described, the fastener 100 is deemed not to conform to the specification. Otherwise, if the fastener 100 has, for example, maximum pin protrusion 110 that is within specification as indicated by the gages, the fastener 100 is deemed to conform to the specification. Thus, the prior art gages may indicate whether or not a fastener is within specifications, i.e., a Go-No Go indication, but, in general, do not provide a direct quantitative measurement of the actual dimension of fastener 100 that corresponds to, for example, the maximum pin protrusion 110 dimension that is indicated on FIG. 1. Thus, the prior art gages have one shortcoming that they cannot be used to provide useful statistical analyses for quality control.
Pin protrusions may also be measured in the prior art using a depth micrometer. It has been found, however, that because of the limited access to confined spaces, such as in the tail cone of a commercial aircraft, that a depth micrometer cannot be used for making the required pin protrusion measurements.
In confined spaces and relatively inaccessible locations often encountered in the manufacture of aircraft, a user of the prior art hand-held gages may have difficulty using the gage in the confined space in a manner in which the gage is designed to be used. The confined space or relatively inaccessible location may not provide an adequate view of the gage for visual alignment, or the view may be blocked by the user's hand holding the gage, so that the gage may have to be used “by feel.” Such use by feel is usually less accurate than visual verification of the gage fit, and may not be possible if, for example, washer 106 does not project sufficiently far from underneath nut 104 to provide a mechanical alignment of the gage.
As can be seen, there is a need for a tool for measurement of pin protrusion of fastenings, such as nut and bolt fastenings used on aircraft. Also there is a need for accurate visual measurement of pin protrusion for nut and bolt fastenings located in confined spaces and relatively inaccessible locations—such as the tail cones of commercial jet liners. Moreover, there is a need for accurate quantitative measurements of pin protrusion of nut and bolt fastenings that may be used to perform statistical analyses and improve quality control for aircraft manufacture.